Twitter Hatches a TV Show

ABC Family has about 1.9 million reasons to be confident that "Pretty Little Liars" is ripe for a spinoff series, the channel's first. That was the number of messages about the hit show that flooded Twitter during a season finale in August, which set a standing record for the top-tweeted episode of any series on television.

The online gusto that propelled ratings of "Pretty Little Liars," a feisty mystery series aimed at girls, also contributed to its creators' choice of which heartthrob character to...